Its structure could provide clues that improve other catalysts.

An engineered enzyme is the first single biological catalyst that converts carbon dioxide into a renewable form of energy: methane. Surprisingly, the same enzyme can use carbon dioxide to make an important ingredient in plastics as well.

Recycling carbon dioxide by turning it into fuels like methanol (CH3OH) or methane (CH4) might be one way to slow the CO2 accumulation in our atmosphere. But that's quite a challenge, because CO2 is a pretty inert molecule and doesn't readily participate in chemical reactions. So chemists have developed metal-containing catalysts to assist in the reduction reactions that convert it to methane and other carbon-containing small molecules. Alternatively, bacteria can use CO2 to make methane, but they use a series of proteins to catalyze the transformation.

Lance Seefeldt at Utah State University and his colleagues study a bacterial enzyme called a nitrogenase, which reduces nitrogen gas (N2) to ammonia (NH3) with the help of a cluster of iron and molybdenum atoms buried inside the protein. The reduction of carbon dioxide to methane requires a transfer of eight electrons, just as ammonia production does, so the scientists wondered if an altered version of this enzyme could accept and reduce carbon dioxide.

They changed two amino acids in one subunit of this protein. The altered nitrogenase converts carbon dioxide to methane for 20 minutes and then slows down. The enzyme’s reaction rate and the number of reactions it catalyzes are comparable to similar soluble metal catalysts.

But the real surprise to Seefeldt was that the enzyme triggered a more complex reaction: it combined two molecules, carbon dioxide and acetylene, to form propylene, a three-carbon ingredient in many plastics. That particular reaction is new for any catalyst, inorganic or biological, he says.

The scientists want to test other versions of the enzyme to see if it can use CO2 to build other kinds of molecules, too. Enzymes are used as biocatalysts to make some chemicals on an industrial scale, but that’s not Seefeldt’s ultimate goal in engineering this enzyme. He wants to extend this enzyme’s catalytic ability to better understand how the protein works.

Lessons about how the binding site environment helps catalyze a particular reaction might translate into clues that help other scientists build better catalysts for the production of methane and other commercially relevant chemicals. This altered enzyme won’t solve our carbon dioxide or energy problems on its own. But its structure, or that of its yet-to-be-found mutant cousins, might provide some useful hints that do help us address those issues by recycling CO2 through chemistry.

Promoted Comments

I think that people are missing the point in their eagerness to downplay the immediate utility of this enzyme for energy purposes. With a very simple change to an existing enzyme, these researchers created a new enzyme for a reaction that, to the best of our knowledge, has never been enzymatically catalyzed before.

This is unbelievably cool, because protein engineering is such an incredibly hard field that it's difficult to call it engineering at this point, it's mostly guess and check with enormous compute clusters and a huge dose of intuition from highly trained and extremely smart structural biologists. This enzyme bodes well for future engineering of new reactions. While it's somewhat interesting to observe the energetics required for this, one has to keep in mind that efficiency hasn't been optimized or even attempted to be optimized; the news is that the reaction is happening at all!

Catalyst or no, you're going to require more energy to perform this reaction than you can liberate from combusting methane back into CO2 (or else we have a perpetual motion machine of the 1st Law type).

The altered nitrogenase converts carbon dioxide to methane for 20 minutes and then slows down.

It's been a while since I've taken chemistry so forgive me if I'm missing something. Why is this the case? Is it related to the average number of catalyzed reactions it can perform before the enzyme starts to break down? It was my understanding that a catalyst lowered the energy barrier of a particular reaction but in this case it seems that the enzyme in question can catalyze various reactions. If this enzyme is capable of catalyzing multiple reactions, are those reactions detrimental to the enzyme itself?

Catalyst or no, you're going to require more energy to perform this reaction than you can liberate from combusting methane back into CO2 (or else we have a perpetual motion machine of the 1st Law type).

Yes, but if you can convert CO2 to methane in any location on Earth, that means you can do so near some efficient power production system (nuclear, tidal, solar, wind), store the methane, and ship it to someplace that can't utilize quite so efficient a power system (or simply use the stored energy for cooking or heating homes during the winter).

"An engineered enzyme is the first single biological catalyst that converts carbon dioxide into a more potent form of greenhouse gas: methane"

fixed

Are you assuming they would just set this stuff out in the open and let the enzyme vent perfectly good fuel into the atmosphere? That would be ludicrous, but I do understand the knee-jerk reaction to the word 'methane'.

Now, if they were to collect it and use it as fuel - combust it - you'd get this:

The altered nitrogenase converts carbon dioxide to methane for 20 minutes and then slows down.

It was my understanding that a catalyst lowered the energy barrier of a particular reaction but in this case it seems that the enzyme in question can catalyze various reactions. If this enzyme is capable of catalyzing multiple reactions, are those reactions detrimental to the enzyme itself?

Your understanding of the mechanism of enzyme catalysis is correct.

I assume the apparently irreversible inactivation of the recombinant enzyme is due to damage to the MoFe cluster by radicals created by side-reactions during catalysis. These reactions seem to be relatively unfavourable kinetically as the inactivation is a slow process.

Some tiny detail missing: What about the law of conservation of energy?

It can't be that you release energy by burning methane and then simply convert the CO2 back into Methane.

So where does the Energy for this reaction come from. If it's just heat at ~40°C, then you've got your practical perpetuum mobile. But more likely the energy comes from the proteins used up, so the input for this desirable reaction more valuable. This makes the option economically less feasible.

This species released to the open world would metabolize CO2 into the way more heating Methane and bring Doom upon the Climate

Catalyst or no, you're going to require more energy to perform this reaction than you can liberate from combusting methane back into CO2 (or else we have a perpetual motion machine of the 1st Law type).

I think the advantage here is that we can store the energy, move it around... think of it like a battery: You always lose energy when it's converted into battery storage, but that's a cost which can be endured for the pure sake of utility.

Some tiny detail missing: What about the law of conservation of energy?

The nitrogen reducing reaction apparently requires 16 ATP for each molecule of N2 split. Sorry, we assumed people know that biology wasn't magic, and didn't violate the conservation of energy.

Opethbass wrote:

This species released to the open world would metabolize CO2 into the way more heating Methane and bring Doom upon the Climate

No, probably not. Unless it had an actual use for the methane this produced, it would quickly just end up with the gene silenced or deleted. Bacteria are extremely sensitive to any unnecessary use of energy.

Catalyst or no, you're going to require more energy to perform this reaction than you can liberate from combusting methane back into CO2 (or else we have a perpetual motion machine of the 1st Law type).

Sure, but this is an enzyme we're talking about. Nitrogenase uses energy from ATP, and I highly doubt this enzyme works differently. Get the modified gene into some cyanobacteria (photosynthetic bacteria) and work on getting this running on solar power in some big pools. The engineering challenge is tough (for biologists and engineers alike) but it could prove useful, whether you use it to make plastics or methane.

Some tiny detail missing: What about the law of conservation of energy?

The nitrogen reducing reaction apparently requires 16 ATP for each molecule of N2 split. Sorry, we assumed people know that biology wasn't magic, and didn't violate the conservation of energy.

Opethbass wrote:

This species released to the open world would metabolize CO2 into the way more heating Methane and bring Doom upon the Climate

No, probably not. Unless it had an actual use for the methane this produced, it would quickly just end up with the gene silenced or deleted. Bacteria are extremely sensitive to any unnecessary use of energy.

Thanks for the clarification, good details. 16 ATP actually semms to be lots of stuff for converting only 1 Molecule CO2. But at least ATP is a source of energy which can be provided in microorganisms i guess. So in theory you can actually use the process.

Edit: ATP releases a maximum of 64kJ/mol due to wikipedia. By comparison burning Methane releases ~3.5kJ/mol (quick calculus on wolframalpha). This results in a efficiency of 3,5/(16*64) = 0,34%. Not so good anymore...ecpecially when considering that ATP is a quite valuable form of Energy in Cells.

If it could use latent energy from what is around it like hot water from a cooling tower or something then it would be a great thing. Sun energy might allow for good amounts photo voltaic energy, might improve from the solar cell. For making things polypropylene it would be a small good thing, still need the energy input but maybe can be made when there is excess energy? Or is more efficient than our current method.

Nitrogenase uses a considerable number of ATPs and reducing power to convert nitrogen to ammonia. I assume the modified nitrogenase requires the same. Hence this isn't a free lunch. The possibility is however that you drive this from some other energy source and would be a way to fix CO2 for conversion into molecules that industry needs. Note, you fix CO2 and say burn methane to release CO2. The net effect is zero on the carbon cycle.

Opethbass wrote:

Some tiny detail missing: What about the law of conservation of energy?

It can't be that you release energy by burning methane and then simply convert the CO2 back into Methane.

So where does the Energy for this reaction come from. If it's just heat at ~40°C, then you've got your practical perpetuum mobile. But more likely the energy comes from the proteins used up, so the input for this desirable reaction more valuable. This makes the option economically less feasible.

This species released to the open world would metabolize CO2 into the way more heating Methane and bring Doom upon the Climate

Sure it does, as a cryogenic liquid. There are arguments to be made for sub-cooled liquid propane as a superior fuel for terrestrial rockets, but liquid methane is close in terms of density-Isp and has much greater potential for rockets launched from Mars. Long-chain hydrocarbons such as kerosenes are initially attractive due to their higher density, but they leave carbon deposits inside fuel-rich pre-burners, which makes them less suitable for reusable engines and/or staged-combustion engines. Light hydrocarbons don't foul up the turbopumps and avoid the complex metallurgical issues with oxidizer-rich designs.

Cool way of protein engineering, though! I remember learning about the many chaperones (I think more than 5!) involved in assembling the Nitrogenase. It's so awesome to see that we can modify such a complicated system to our needs!

Some tiny detail missing: What about the law of conservation of energy?

The nitrogen reducing reaction apparently requires 16 ATP for each molecule of N2 split. Sorry, we assumed people know that biology wasn't magic, and didn't violate the conservation of energy.

Opethbass wrote:

This species released to the open world would metabolize CO2 into the way more heating Methane and bring Doom upon the Climate

No, probably not. Unless it had an actual use for the methane this produced, it would quickly just end up with the gene silenced or deleted. Bacteria are extremely sensitive to any unnecessary use of energy.

Thanks for the clarification, good details. 16 ATP actually semms to be lots of stuff for converting only 1 Molecule CO2. But at least ATP is a source of energy which can be provided in microorganisms i guess. So in theory you can actually use the process.

16 ATP is an enormous energy cost for a reaction producing a non-metabolically useful molecule (i.e. methane). Recall that oxidation of one molecule of glucose during regular (aerobic) respiration has a maximum yield of 38 ATP. This is going to confer a significant fitness penalty to the organism under non-lab conditions.

"An engineered enzyme is the first single biological catalyst that converts carbon dioxide into a more potent form of greenhouse gas: methane"

fixed

Are you assuming they would just set this stuff out in the open and let the enzyme vent perfectly good fuel into the atmosphere? That would be ludicrous, but I do understand the knee-jerk reaction to the word 'methane'.

Now, if they were to collect it and use it as fuel - combust it - you'd get this:

CH4 + 2 O2 → CO2 + 2 H2O

Back to carbon dioxide and some water (net results).

I wasn't being serious with my comment. But there has been problems with methane leaks during transmission and storage that one needs to be careful, as methane is so much more potent as a greenhouse gas.

16 ATP is an enormous energy cost for a reaction producing a non-metabolically useful molecule (i.e. methane). Recall that oxidation of one molecule of glucose during regular (aerobic) respiration has a maximum yield of 38 ATP. This is going to confer a significant fitness penalty to the organism under non-lab conditions.

So finally under good conditions it's 1 sugar for 2 methane. Not good at all for producing methane. But the scientists are right, metabolising sugar ("grows" in nature) into plastics may indeed be useful.

I know very little about how methane can best be stored, but years ago there was a dairy farmer near us in Minnesota who capped his large liquid manure tank and used the methane to run a generator that supplied much of his electrical needs. I always thought that was pretty cool.

16 ATP is an enormous energy cost for a reaction producing a non-metabolically useful molecule (i.e. methane). Recall that oxidation of one molecule of glucose during regular (aerobic) respiration has a maximum yield of 38 ATP. This is going to confer a significant fitness penalty to the organism under non-lab conditions.

So finally under good conditions it's 1 sugar for 2 methane. Not good at all for producing methane. But the scientists are right, metabolising sugar ("grows" in nature) into plastics may indeed be useful.

This is a rather moot point anyway, as the enzyme is used in a purified form in the PNAS study. (This isn't a bioreactor experiment).

I think that people are missing the point in their eagerness to downplay the immediate utility of this enzyme for energy purposes. With a very simple change to an existing enzyme, these researchers created a new enzyme for a reaction that, to the best of our knowledge, has never been enzymatically catalyzed before.

This is unbelievably cool, because protein engineering is such an incredibly hard field that it's difficult to call it engineering at this point, it's mostly guess and check with enormous compute clusters and a huge dose of intuition from highly trained and extremely smart structural biologists. This enzyme bodes well for future engineering of new reactions. While it's somewhat interesting to observe the energetics required for this, one has to keep in mind that efficiency hasn't been optimized or even attempted to be optimized; the news is that the reaction is happening at all!

Edit: ATP releases a maximum of 64kJ/mol due to wikipedia. By comparison burning Methane releases ~3.5kJ/mol (quick calculus on wolframalpha). This results in a efficiency of 3,5/(16*64) = 0,34%. Not so good anymore...ecpecially when considering that ATP is a quite valuable form of Energy in Cells.

I'm getting numbers around of 800-900 kJ/mol for methane combustion both from Wikipedia 12 and from Wolfram Alpha, which would make this enzyme strikingly efficient if it were just 16 ATP (however, I don't fully understand these terms, so if somebody more knowledgeable would correct me, I'd be appreciative). However, the 16 ATP is just a guess based on the nitrogenase activity, the precise energy consumption would need to be measured for this to be an interesting number.

As I said above, the energy consumption is not really the point of this paper; perhaps the next paper will tell us more.

This seems really irresponsible. If geophysists decide to engineer an organism to utilize this enzyme, which they inevitably are trying to do, what if it thrives? Have we learned nothing from the lessons of invasive species control? You bring in one species to correct the problems of another, and almost without exception it exacerbates the problem greatly. Meddling with the composition of the atmostphere can bring nothing but trouble.

This seems really irresponsible. If geophysists decide to engineer an organism to utilize this enzyme, which they inevitably are trying to do, what if it thrives? Have we learned nothing from the lessons of invasive species control? You bring in one species to correct the problems of another, and almost without exception it exacerbates the problem greatly. Meddling with the composition of the atmostphere can bring nothing but trouble.

16 ATP is an enormous energy cost for a reaction producing a non-metabolically useful molecule (i.e. methane). Recall that oxidation of one molecule of glucose during regular (aerobic) respiration has a maximum yield of 38 ATP. This is going to confer a significant fitness penalty to the organism under non-lab conditions.

So finally under good conditions it's 1 sugar for 2 methane. Not good at all for producing methane. But the scientists are right, metabolising sugar ("grows" in nature) into plastics may indeed be useful.

Unfortunately, to get the energy from one glucose molecule, you have to burn it, releasing 6 CO2 molecules.

This is more about the principle that in the long run, our hydrocarbon use will become renewable. Eventually we will burn it all, we are just haggling about timeframes.

When that time comes, if we want liquid hydrocarbons (eg for jet fuel), it will have to be synthesized, at a net energy loss. And given how energy dense hydrocarbons are, we will probably pay that price to have them available.

At the moment, we are still trying to work out how to reverse the very simple process of burning fossil fuels, and this is as good an approach as any out there.

"But the real surprise to Seefeldt was that the enzyme triggered a more complex reaction: it combined two molecules, carbon dioxide and acetylene, to form propylene, a three-carbon ingredient in many plastics."

Hooray! Now we can convert all the excess CO2 into plastic bags and the world will be saved!

Unfortunately, to get the energy from one glucose molecule, you have to burn it, releasing 6 CO2 molecules.

This is more about the principle that in the long run, our hydrocarbon use will become renewable. Eventually we will burn it all, we are just haggling about timeframes.

When that time comes, if we want liquid hydrocarbons (eg for jet fuel), it will have to be synthesized, at a net energy loss. And given how energy dense hydrocarbons are, we will probably pay that price to have them available.

At the moment, we are still trying to work out how to reverse the very simple process of burning fossil fuels, and this is as good an approach as any out there.

Michael

I happen to align with an earlier opinion that this is an incredible find given that it marks the beginning of a new line of useful enzymes. To bucket chemists (chemical engineers), this results in near nirvana. Look to huge funding increases as a result. That methane results is "nice." Methane is a useful precursor in its own right. A plastic precursor? Wow!

As for your time-frame/costs argument, any process of synthesis is going to result in a net energy loss unless you've managed to revoke the laws of thermodynamics. Hell any process on the macro level to be certain. (I hold reservations about the quantum level.) It is all about balancing economic opportunity costs of each particular process in its entirety. I should point out that the jet-fuel argument is specious since you are assuming that current jet engine technologies are the bases of air transport for the future. Methane has already been cited as a probable fuel of choice for hypersonic transport, should that come about, so an additional base is covered for advocating that choice. (Not that we lack for farm animals, humans, or termite mounds for that matter to act as fuel sources ;-).

One pleasant feeling I have from reading the discussion here is that people are actually trying to get a handle on the end-to-end processes that may be involved. Sadly, little innovation that I have observed has those considerations in mind. We are, as a species, trying to be better; still lots of ground to cover to get there.

I happen to align with an earlier opinion that this is an incredible find given that it marks the beginning of a new line of useful enzymes. To bucket chemists (chemical engineers), this results in near nirvana. Look to huge funding increases as a result. That methane results is "nice." Methane is a useful precursor in its own right. A plastic precursor? Wow!

As for your time-frame/costs argument, any process of synthesis is going to result in a net energy loss unless you've managed to revoke the laws of thermodynamics. Hell any process on the macro level to be certain. (I hold reservations about the quantum level.) It is all about balancing economic opportunity costs of each particular process in its entirety. I should point out that the jet-fuel argument is specious since you are assuming that current jet engine technologies are the bases of air transport for the future. Methane has already been cited as a probable fuel of choice for hypersonic transport, should that come about, so an additional base is covered for advocating that choice. (Not that we lack for farm animals, humans, or termite mounds for that matter to act as fuel sources ;-).

One pleasant feeling I have from reading the discussion here is that people are actually trying to get a handle on the end-to-end processes that may be involved. Sadly, little innovation that I have observed has those considerations in mind. We are, as a species, trying to be better; still lots of ground to cover to get there.

I think you have missed the points I was making.

I agree that there is an energy cost - that goes without saying. I am saying that you wouldn't power this with glucose, as you might as well turn the glucose molecule into 6 methane molecules directly. So as long as we are willing to divert food into fuel, its pretty useless too. But it is only a matter of time, and probably not that much time, before we see that making biofuels pushes up the price of food above the level that third world nations can afford. But for as long as we use biofuels, we probably wont use this process to generate methane, as it will make more sense to divert the biofuels to methane production.

Eventually however, we will have to live on renewable resources - its almost definitional. If its non-renewable energy, you only get to use it once.

At that point, we may still want the main other benefit of hydrocarbons: energy density in a safe, compact form. If we want to fly planes, we will want this. I'm not arguing about methane versus jet fuel - the two are equivalent via the Fischer-Tropsch process.

I think you will find jet fuel has better characteristics in terms of remaining liquid between -50 and +40 Celcius, without tending to ignite spontaneously. But if you want to make a methane powered jet plane, I'm not fighting you on that point. Personally I'd just convert the methane to longer chain hydrocarbons, but maybe in the future we can build an engine that runs better on methane.

Some tiny detail missing: What about the law of conservation of energy?

The nitrogen reducing reaction apparently requires 16 ATP for each molecule of N2 split. Sorry, we assumed people know that biology wasn't magic, and didn't violate the conservation of energy.

Opethbass wrote:

This species released to the open world would metabolize CO2 into the way more heating Methane and bring Doom upon the Climate

No, probably not. Unless it had an actual use for the methane this produced, it would quickly just end up with the gene silenced or deleted. Bacteria are extremely sensitive to any unnecessary use of energy.

Thanks for the clarification, good details. 16 ATP actually semms to be lots of stuff for converting only 1 Molecule CO2. But at least ATP is a source of energy which can be provided in microorganisms i guess. So in theory you can actually use the process.

Edit: ATP releases a maximum of 64kJ/mol due to wikipedia. By comparison burning Methane releases ~3.5kJ/mol (quick calculus on wolframalpha). This results in a efficiency of 3,5/(16*64) = 0,34%. Not so good anymore...ecpecially when considering that ATP is a quite valuable form of Energy in Cells.

That still seems to be a good step up from the efficiently of solar panels.

I think that people are missing the point in their eagerness to downplay the immediate utility of this enzyme for energy purposes. With a very simple change to an existing enzyme, these researchers created a new enzyme for a reaction that, to the best of our knowledge, has never been enzymatically catalyzed before.

That's the optimistic view of it.

The pessimistic view of it is that it WAS catalyzed before in primordial soup antiquity and isn't now for some reason.

Nitrogen fixation is energy expensive. Which is presumably why it is only done by a few clades.

This is even worse for methanogen metabolism, where the series of catalysts mentioned has been claimed to be the most energetically demanding enzymes. It is by the way done exclusively by archaea, which are prokaryotes but different from bacteria. [ http://en.wikipedia.org/wiki/Methanogen ]

Interestingly, the two metabolisms choose different solutions.

Methanogens evolved out of aerobic methanophiles, so emerged after the atmosphere oxygenated.

But so did the nitrogen fixation bacteria, since there would be ammonia around locally in a neutral (CO2 dominated) atmosphere. They just don't seem to have had a previous metabolism to adapt a whole sequence from.

So where does the Energy for this reaction come from. If it's just heat at ~40°C, then you've got your practical perpetuum mobile.

As an example, in the methanogens compared with here, they are using the CO2 to scavenge energy out of H2 anaerobically:

"Some methanogens, called hydrogenotrophic, use carbon dioxide (CO2) as a source of carbon, and hydrogen as a reducing agent.

The reduction of carbon dioxide into methane in the presence of hydrogen can be expressed as follows:

CO2 + 4 H2 → CH4 + 2 H2O

Some of the CO2 is reacted with the hydrogen to produce methane, which creates an electrochemical gradient across cell membrane, used to generate ATP through chemiosmosis. In contrast, plants and algae use water as their reducing agent." [ http://en.wikipedia.org/wiki/Methanogen ]

So yes, you need to feed the enzyme with energy in order to make methane, as already noted.